The first plane, a Cessna, entered restricted airspace at 11:10 a.m. local time. Another Cessna flew into the same area eight minutes later. The third incident, involving a Katana, happened at 1:13 p.m.

"All three pilots were questioned but we had no protected interest in holding them," Secret Service spokesman John Gill told CNN. "We have concluded that these incidents were inadvertent."

He also said it is normal procedure for the FAA to conduct a follow-up investigation.

Airport officials at Waco said the planes might have had more difficulty navigating Saturday because of hazy conditions.